French Frigate Shoals has earned a reputation as one of the more challenging sites in the Northwestern Hawaiian Islands – both for monk seals and monk seal camp biologists.
Why? Life can be hard at French Frigate Shoals! Once the home to the largest subpopulation of Hawaiian monk seals, French Frigate Shoals is now the site most consistently showing the highest mortality rates for seals, especially for young seals less than 2 years old. The greatest threat to young seals is one that is entirely unique to French Frigate Shoals. A small number of the Shoal’s Galapagos sharks have developed the peculiar behavior of focusing predation on monk seal pups, often swimming right up to take pups off the sand.
At least 9 of the 35 pups born at French Frigate Shoals this season were lost to Galapagos shark predation. This is not a new problem. And while it is a relatively small number of sharks involved, the impact on the monk seal population is significant. In recent years, more than 250 of the roughly 1000 pups born at French Frigate Shoals have been a victim of Galapagos shark predation. The total number of pups known to be taken by Galapagos sharks in the rest of the Northwestern Hawaiian Islands during the same period is 0.
Over the years, Hawaiian Monk Seal Research Program (HMSRP) field researchers have tried an assortment of mitigation strategies. They focus efforts at the most dangerous islets where Galapagos sharks are observed attacking nursing and weaned pups right at the shoreline. Efforts have included deterrents such as underwater speakers, large magnets, and electric “shark shields” designed for scuba divers. But none have significantly reduced the mortality rate. The team even dedicates part of their field season to fishing for the specific individual sharks that prey on monk seals at the high-risk islets. This selective fishing of a few sharks will likely have greater impact reducing seal mortality than other methods. However, the highly selective nature of the fishing means often only one shark may be caught in a year (and often none). So we have a way to go before we start seeing the benefit of the effort.
In the meantime, vigilant and protective monk seal moms are the best defense for nursing pups. Unfortunately, when mom is out of milk and abruptly weans her pup, the young and naïve seal is left to fend for itself. This is when most shark attacks happen. So field biologists at French Frigate Shoals must remain extra vigilant as well. They carefully track nursing pups, and as soon as a pup weans, they scoop the “weaners” up in a stretcher net and give them a 20 minute boat ride (during which pups usually fall asleep) to Tern Island, which is relatively safe with 0 shark attacks most years.
This video shows a protective female monk seal fending off two Galapagos sharks attempting to prey on her pup at Trig Island, French Frigate Shoals.
This year HMSRP researchers translocated 11 weaned pups to the safety of Tern Island. Unfortunately, upon ending our field season and leaving French Frigate Shoals on August 20th, there were still 6 pups nursing, and more expected to be born at these high-risk islets. For the biologists, it is always difficult to leave knowing there are more seals that would benefit from our help… but we can only do what we can do.
All monk seal work was conducted under NOAA ESA/MMPA permits 16632-01 and/or 18786.